Electric Brake

ABSTRACT

Provided is an electric brake capable of accurately estimating the brake pad temperature and securing a sufficient braking force and response. An electric brake has a disc rotor rotating with a wheel, an actuator for rectilinearly actuating a piston in the axial direction of the disc rotor by using an electric motor, a drive controller for controlling the drive of the actuator, a brake pad pressed by the piston to give a frictional resistance to the disc rotor in the direction of rotation, and a braking start position detector for detecting a braking start position of the piston where the disc rotor is brought into contact with the brake pad. The drive controller stores the braking start position detected by the braking start position detector as the maximum braking start position. When the braking start position shifts in the pressing force-increasing direction, the drive controller updates the value stored as the maximum braking-start position.

TECHNICAL FIELD

The present invention relates to a brake apparatus for an automobile,and particularly to an electric brake apparatus which generates brakingforce using an electric motor.

BACKGROUND ART

Conventionally, there is known development of an electric brakeapparatus in which an electric motor is rotationally driven depending onthe amount of stepping of a brake pedal to generate braking force usingthe rotational torque of the electric motor. For example, the electricbrake apparatus proposed in JP-A-2001-32868 comprises an actuator havingan electric motor, and is adapted to apply braking force to a wheel bypressing a brake pad against a disc rotor depending on the amount ofstepping of a brake pedal.

The electric brake apparatus in the above conventional example uses theposition of the brake pad at the time of releasing thrust detected by athrust sensor when braking is finished as a braking start position;controls a gap between the brake rotor and the brake pad (hereinafterreferred to as a pad gap) so that the brake pad is spaced from thebraking start position by a predetermined amount when the brake isreleased; and controls the thrust of a piston depending on the thrustdetected by the thrust sensor when generating the braking force.

When the brake is released, in order to avoid contact between the discrotor and the brake pad (dragging of the brake) resulting from thermaldeformation of the disc rotor, the temperature of the disc rotor or thetemperature of the brake pad (hereinafter referred to as padtemperature) is detected by a temperature sensor, or the temperature ofthe disc rotor is estimated based on the cumulative value of heat energyof the disc rotor calculated based on the vehicle speed, the outside airtemperature and a braking state, so that the brake pad is spaced fromthe disc rotor depending on the amount of thermal deformation of thedisc rotor.

DISCLOSURE OF THE INVENTION

As seen in the above conventional example, in an electric brakeapparatus, the pad gap and the thrust change in accordance with changein temperature of the brake disc and the brake pad. If the braking isreleased, the pad gap is increased due to thermal contraction of thebrake pad in connection with cooling thereof, and wasteful time untilstarting the braking will be increased. In addition, if the temperatureof the brake pad rises to reduce the hardness (rigidity) of the brakepad during braking, the amount of rotation of the motor for generatingpredetermined braking force increases and therefore responsiveness ofgenerating the braking force is degraded. Further, if the temperature ofthe brake pad changes, the coefficient of friction between the brake padand the disc rotor changes, and therefore the degree of deceleration ofthe vehicle may vary even when the brake pad is pressed with the samethrust. Furthermore, if the temperature of the brake pad is loweredduring parking braking and the thermal contraction is caused thereby,the thrust decreases, and therefore, the parking brake needs to be setwith great braking force in advance in order to compensate the decreaseof the thrust, which promotes increase of the electric power consumptionand wear of mechanical components. For securing sufficient braking forceand responsiveness against these changes of the pad gap and the thrustresulting from change in temperature of the brake pad, it is necessaryto grasp the temperature of the brake pad accurately and adjust thecontrol of braking force depending on the temperature change and thethermal expansion.

However, the temperature detection by a temperature sensor in the aboveconventional art leads to complication of the structure and increase insize of the actuator. Further, if a temperature sensor is assembled inthe disc rotor and the brake pad which are expendable parts, there isthe problem that not only manufacturing cost, but also maintenance costincrease.

On the other hand, in the case of a temperature estimation methodutilizing heat energy balance as in the above conventional art, it isdifficult to estimate the pad temperature accurately since the heatenergy balance is affected not only by outside air temperature andvehicle wind, but also by natural wind, road surface temperature and thelike. Particularly when the electric brake apparatus is left for a longtime after its power source is turned off, the estimation errorincreases, which makes it more difficult to grasp the heat energybalance accurately. Accordingly, it is difficult to estimate the padtemperature with good accuracy under all circumstances by thetemperature estimation method based on the heat energy balance.

Therefore, the present invention has been made in view of the aboveproblem, and its object is to provide an electric brake apparatus whichcan estimate the temperature of the brake pad with good accuracy withoutusing a temperature sensor and can secure sufficient braking force andresponsiveness even in a situation where the temperature of the brakepad varies.

The above object is achieved by a drive controller storing a brakingstart position detected by a braking start position detecting means as amaximum braking start position, and updating the value of the maximumbraking start position when the braking start position shifts in apressing force increasing direction or when replacement of the brake padis detected by a pad replacement detecting means. In this way, theamount of thermal expansion and the amount of wear of the brake pad canbe estimated by comparing the maximum braking start position with thebraking start position at a high temperature and the braking startposition after replacement of the pad, and it becomes possible toprovide an electric brake apparatus which can secure sufficient brakingforce and responsiveness even in a situation where the temperature ofthe brake pad varies.

In the electric brake apparatus, it is preferable that the drivecontroller utilizes the braking start position detected by the brakingstart position detecting means as a current braking start position,updates it in a period shorter than the maximum braking start position,and calculates the amount of thermal expansion of the brake pad based onthe difference between the maximum braking start position and thecurrent braking start position. In this way, the amount of thermalexpansion of the brake pad can be detected with good accuracy wheneverthe braking start position is detected, and if control of the brakingforce is performed depending on the amount of thermal expansion, it ispossible to provide an electric brake apparatus which can securesufficient braking force and responsiveness even in a situation wherethe temperature of the brake pad varies.

In addition, it is preferable that the drive controller stores thebraking start position at the beginning of wear of the brake pad or atthe time of replacing the brake pad as an initial braking startposition, and calculates the amount of wear of the brake pad based onthe difference between the maximum braking start position and theinitial braking start position. In this way, the amount of wear of thebrake pad can be detected whenever the maximum braking start position isupdated, and if control of the braking force is performed depending onthe amount of wear, it is possible to provide the electric brakeapparatus which can secure sufficient braking force and responsivenesseven in a situation where the amount of wear of the brake pad varies.

In addition, it is preferable that the drive controller changes thethrust of the piston or the position of the piston based on thedifference between the maximum braking start position and the currentbraking start position, or the difference between the maximum brakingstart position and the initial braking start position. In this way, itis possible to provide the electric brake apparatus which can securesufficient braking force and responsiveness even in a situation wherethe thrust of the piston or the coefficient of friction of the brake padvaries due to changes of the temperature or the amount of wear of thebrake pad.

In addition, it is preferable that the drive controller changes thethrust at the time of starting a holding operation of the parking brakemechanism based on the amount of thermal expansion or the amount of wearof the brake pad. In this way, it is possible to provide the electricbrake apparatus which can secure necessary and sufficient braking forceof the parking brake even in a situation where the temperature or theamount of wear of the brake pad varies after the parking brake is set.

In addition, it is preferable that the drive controller changes theposition of the piston at the time of starting a holding operation ofthe parking brake mechanism based on the maximum braking start position.In this way, it is possible to provide the electric brake apparatuswhich can secure necessary and sufficient braking force of the parkingbrake even in a situation where the temperature or the amount of wear ofthe brake pad varies after the parking brake is set.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a flow chart of the present invention;

FIG. 2 is a view showing an embodiment of the present invention;

FIG. 3 is a diagram showing a braking start position of the presentinvention;

FIG. 4 is a diagram showing a braking start position of the presentinvention;

FIG. 5 is a diagram showing a parking brake state of the presentinvention;

FIG. 6 is a diagrams showing a state estimation of the presentinvention; and

FIG. 7 is a diagram showing a correction operation in thrust control ofthe present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows an embodiment of a brake apparatus of the presentinvention.

The brake apparatus comprises a brake pedal 1, a pedal sensor 2 fordetecting the stepping amount of the brake pedal, an operating statedetection unit 3, a vehicle motion controller 4 for calculating brakingforce, an electric power supply 5, an actuator 6 which is an electricbraking force generating mechanism, a drive controller 7 for driving theactuator 6 and for transmitting and receiving a signal, and a caliper 8.

The actuator 6 comprises a housing 9, a motor 11, a stator 12 which is afixing part of the motor 11 to the housing 9, a motor rotor 13 which isa rotating part of the motor 11, a screw portion 14 for convertingrotation of the motor rotor 13 to linear motion, a bearing 15 supportingthe motor rotor 13, a piston 16 generating thrust from the rotationalpower of the motor rotor 13, a parking brake mechanism 20 formechanically fixing the position of the motor rotor 13 and the piston 16by hooking a plunger of a solenoid in a groove of the motor rotor 13,and two brake pads 21 which receive the thrust of the piston 16 to pincha disc rotor 22.

The actuator 6 and the brake pad 21 are fixed to the floating typecaliper 8. The caliper 8 is supported so as to be slidable in an axialdirection of the motor 11 (the lateral direction in the drawing) withrespect to an axle fixed portion interlocked with the movement of asuspension and a steering. The disc rotor 22 rotating together with atire is arranged between the two brake pads 21. Frictional force isgenerated between the brake pads 21 and the disc rotor 22 by the thrustof the piston 16, and is transmitted to a road surface through the tireto generate braking force on each wheel.

The actuator 6 is provided with a rotation angle sensor 31 for detectingthe rotation angle of the motor 11, and a thrust sensor 32 for detectingthe thrust of the piston 16 a signal value of which varies depending onthe braking force variation. The rotation angle sensor 31 is, forexample, a Hall element, an encoder or a resolver. The thrust sensor 32is, for example, a strain gauge-type load cell. The analog signalsoutputted from these sensors are sent to the drive controller 7 througha signal line 41 connecting each sensor and the drive controller 7.

The electric power supply 5 and the drive controller 7 are connected bya power line 42 so that electric power is supplied for driving the drivecontroller 7 and the motor 11. The vehicle motion controller 4 and thedrive controller 7 are connected by the signal line 41. The signal line41 transmits a signal from the vehicle motion controller 4 to the drivecontroller 7, while transmitting information from multiple sensorsprovided on the drive controller to the vehicle motion controller 4.

Hereinafter, description will be made to the operation of the brakeapparatus having the above described configuration.

The pedal sensor 2 outputs an electric signal depending on the steppingamount of the brake pedal 1. The operating state detection unit 3detects, for example, the vehicle speed, the vehicle acceleration, theturning angular speed of the vehicle, the stepping amount of anaccelerator pedal by a driver, the engine throttle opening degree, thesteering angle of a steering gear, the distance from and relative speedto a preceding car, the presence of an obstacle, or the road grade, andsends the electric signal depending on the respective states to thevehicle motion controller 4. Actuator information such as a motorrotation angle and a piston thrust is sent from the drive controller 7to the vehicle motion controller 4.

The vehicle motion controller 4 calculates a braking force requirementvalue for each wheel based on signals from the pedal sensor 2, theoperating state detection unit 3, and the drive controller 7, andconverts it into a target piston thrust. The vehicle motion controller 4sends a signal depending on the amount of the target piston thrust tothe drive controller 7. When a braking force is requested by the vehiclemotion controller 4, the drive controller 7 controls the motor 11 sothat a sensor signal value from the thrust sensor 32 is brought into thetarget piston thrust. When the motor rotor 13 is rotated in a direction(i.e. in a forward direction) so that the piston 16 advances to the padside (in a right-hand direction in FIG. 2), the thrust is increased.When the motor rotor 13 is rotated in a direction (i.e. in a reversedirection) so that the piston 16 is retracted, the thrust is decreased.If clearance is generated between the brake pad 21 and the disc rotor22, the piston thrust becomes zero and the braking force is released.

When the vehicle motion controller 4 generates a request signal forparking brake, the drive controller 7 controls the motor 11 until asensor signal value from the thrust sensor 32 reaches a predeterminedpiston thrust value, and the parking brake mechanism 20 mechanicallylocks the rotation of the motor rotor 13 when the sensor signal valuereaches the predetermined piston thrust value. After the braking forceis maintained by the parking brake mechanism 20, the braking force isstill maintained even after power to the motor 11 is cut.

When the braking force request value is zero, namely, when the brakingforce is released with no request for piston thrust generation, thedrive controller 7 controls the motor 11 to reduce the piston thrust tozero. In order to avoid pad dragging due to contact between the brakepads 21 and the disc rotor 22, a predetermined gap is provided betweenthe brake pads 21 and the disc rotor 22. In the position control for thepad gap, the motor 11 is controlled so that the disc rotor 22 isseparated by a predetermined distance relative to a braking startposition where the brake pads 21 and the disc rotor 22 are in contactwith each other.

As shown in FIG. 3, the braking start position X0 is determined bysubtracting a predetermined amount ΔXα from a piston position where thepiston thrust F becomes a predetermined threshold value Fth when thebraking force is generated or released. The braking start position X0may be a detected value in every detection, or a calculated value basedon a plurality of detected values, such as an average detected valuebased on a number of past detections.

As shown in FIG. 4, the drive controller 7 stores three types of brakingstart positions detected in different times (i.e. a current brakingstart position X0crnt, an initial braking start position X0init, and amaximum braking start position X0max), and determines a wear amountΔXwear (hereinafter, referred to simply as a wear amount), and a thermalexpansion amount ΔXthrm (hereinafter referred to simply as a thermalexpansion amount) of the brake pad 21 on the basis of these brakingstart positions.

The current braking start position is updated whenever a braking startposition is determined by a detection operation. This means that thecurrent braking start position is updated in the shortest period, andshows the latest detected braking start position.

The maximum braking start position is updated only when a newly detectedbraking start position is larger than the maximum braking start positionstored at that time (which is positive in a wear advance direction), orwhen the brake pad 21 is replaced. This means that the maximum brakingstart position shows a braking start position when a detected positionis largest in the course of wear on a particular brake pad 21. Inaddition, the brake pads 21 contract to reduce their thickness as thetemperature decreases, so that the braking start position is increased.The outside air temperature (approximately −40° C. to +40° C.) is lowerthan the temperature of a braked pad (+100° C. or higher). If thebraking start position is detected in a state where the pad temperatureis approximately the same as the outside air temperature, for example,when the vehicle is started, the maximum braking start position reflectsthe progress of wear of the brake pads 21, and the braking startposition at a low temperature will be detected.

As an initial braking start position, the braking start positionimmediately after assembling the brake pads 21 is stored. For example,the stored value is updated at the time of producing a product, or atthe time of a replacement operation of brake pad replacement. At thistime, since the pad temperature is approximately the same as the outsideair temperature, the initial braking start position is in an initialstate before wear of the brake pads 21 progresses, and is a brakingstart position when its temperature is equal to the outside airtemperature.

The drive controller 7 estimates the amount of wear and the amount ofthermal expansion of the brake pads 21 from the three types of brakingstart positions detected in different times. The amount of wear can bedetermined from the difference between the initial braking startposition and the maximum braking start position. Both of the initialbraking start position and the maximum braking start position representthe braking start positions at the outside air temperature, andtherefore the difference in temperature of the pad at the time ofdetection has less effect on the difference in thickness of the pad.Accordingly, the difference between the initial braking start positionand the maximum braking start position corresponds to the amount of wearof the brake pads 21. The amount of thermal expansion is determined fromthe difference between the current braking start position and themaximum braking start position. In contrast to the maximum braking startposition, the current braking start position after braking control mayincrease in thickness due to the increased temperature of the pads.Since the progressing rate of pad wear is considerably slower thanchange of the thermal expansion amount, the difference between thecurrent braking start position and the maximum braking start position isconsidered to be caused by the thermal expansion of the brake pads 21.

If there is a relatively large difference between the outside airtemperature when the maximum braking start position is updated and theoutside air temperature during braking, the amount of thermal expansionestimated based on the difference between the maximum braking startposition and the current braking start position does not accurately showthe amount of thermal expansion at the outside air temperature duringbraking. In this case, if it is possible to determine at what outsideair temperature the maximum braking start position has been updated, theamount of thermal expansion at the outside air temperature duringbraking can be accurately estimated by correcting the difference betweenthe maximum braking start position and the current braking startposition depending on the outside air temperature during update. Forexample, if an updated value is larger than a value before the update bya predetermined value or more when updating the maximum braking startposition, it can be estimated that the position has been updated at alower outside air temperature than the previous update. Further, if astate where the difference between the maximum braking start positionand the current braking start position is equal to or beyond apredetermined value when updating the current braking start positioncontinues for more than a predetermined number of times, it can beestimated that the maximum braking start position has been updated at alow outside air temperature. In addition, the outside air temperatureduring update can be detected directly by an outside air temperaturesensor. If it is possible to determine at what outside air temperaturethe maximum braking start position has been updated in this way, thedifference between the maximum braking start position and the currentbraking start position can be corrected depending on the outside airtemperature during update, and the amount of thermal expansion of padsat the outside air temperature during braking can be more accuratelydetermined.

By determining the amount of thermal expansion of the brake pads 21 bycomparing braking start positions detected at different timings in thisway rather than estimation of a thermal energy balance, it is possibleto accurately determine the amount of thermal expansion of pads withoutbeing affected by an outside condition nor an idling period after thepower is turned off.

Hereinafter, a method for estimating the amount of thermal expansion andthe amount of wear will be described with reference to a flow chartshown in FIG. 1. At S1, determination is made whether the piston thrusthas passed a threshold value Fth which is a reference value fordetecting the braking start position. If a value less than zero isyielded by subtracting the threshold value Fth from each of the thrustsensor value F[t] at the current time t and the thrust sensor valueF[t−1] at the previous calculation step and then multiplying the resultsby each other, it is determined that the piston thrust has passed thethreshold value. If it has passed the threshold value, the processproceeds to S2, and otherwise, the process is suspended for a while andresumes the similar processing at the next calculation step. At S2, atemporary braking start position X0temp is calculated. An average valueis determined between the piston position X[t] at the current time t andthe piston position X[t−1] at the previous calculation step, from whicha predetermined displacement ΔXα is subtracted to yield the temporarybraking start position X0temp. At S3, update is made using the temporarybraking start position X0temp as the current braking start positionX0crnt. At S4, determination is made whether the temporary braking startposition X0temp is larger than the maximum braking start position X0max,or whether it is immediately after assembling the brake pads. If it istrue, the process proceeds to S5, and if not, the process proceeds toS9. At S5, update is made using the temporary braking start positionX0temp as the maximum braking start position X0max. At S6, determinationis made whether it is immediately after assembling the brake pads. If itis true, the process proceeds to S7, and if not, the process proceeds toS8. At S7, update is made using the temporary braking start positionX0temp as the initial braking start position X0init. At SB, thedifference between the initial braking start position X0init and themaximum braking start position X0max is determined as the wear amountΔXwear, and the process proceeds to S9. At S9, the difference betweenthe current braking start position X0crnt and the maximum braking startposition X0max is determined as the thermal expansion amount ΔXthrm, andthe process proceeds to the next calculation step.

If the amount of thermal expansion can be estimated, the parking brakeoperation on the assumption that the braking force decreases due to thethermal contraction can be realized. When the amount of thermalexpansion is large, the necessary braking force to deal with thedecrease of braking force due to the thermal contraction can be securedby setting the thrust of the piston to be greater than necessary holdingbraking force. For example, in the case where a current braking startposition is different from the maximum braking start position and it isthought that the brake pad 21 is thermally expanded as shown in FIG. 5,if the braking force is held at point (a), the braking force to be helddecreases to point (b) after the pad temperature lowers, and thenecessary braking force cannot be secured. However, the amount ofthermal expansion can be estimated based on the difference between thecurrent braking start position and the maximum braking start position.If the braking force is held at point (c) by predicting the thermalcontraction of the brake pad 21, the braking force can be secured beyondthe necessary braking force to be held since the braking force to beheld shifts to point (d) after the thermal contraction. In addition,when the amount of thermal expansion is small, by setting a relativelylow piston thrust such as the point (d), it becomes unnecessary to setthe parking brake with stronger braking force than required, resultingin reduced power consumption and reduced load on the mechanism. Althoughthe braking force to be held has been considered as an object to becontrolled in the parking brake operation, a piston position to be heldmay be considered as an object. In this case, regardless of thermalexpansion and thermal contraction, the braking force to be held at thepoint (d) can be secured by setting the piston position relative to themaximum braking start position.

The drive controller 7 estimates the temperature, the coefficient offriction, and the rigidity characteristic (the characteristic of thepiston thrust with reference to the piston displacement) of the brakepad 21 based on the amount of thermal expansion and the amount of wearof the brake pad 21. The drive controller 7 stores the relation amongthe amount of thermal expansion, the amount of wear, and the padtemperature in advance, and estimates the pad temperature based on theamount of thermal expansion and the amount of wear. For example, the padtemperature can be estimated by storing characteristics as shown in FIG.6-(A) in a theoretical or experimental way. If the wear of the padprogresses, the amount of thermal expansion caused by a certaintemperature change relatively becomes small because of the reduction ofthe pad thickness. In addition, while the coefficient of friction of thebrake pad 21 varies mainly depending on the pad temperature, thecoefficient of friction of the brake pad 21 can be estimated by storingthe relation between the temperature and the coefficient of frictionthereof in advance as shown in FIG. 6-(B). Since the coefficient offriction of the brake pad 21 varies intricately depending on materials,the characteristic change thereof needs to be determined experimentally.Further, while the rigidity characteristic of the brake pad 21 variesmainly depending on the pad temperature and the amount of wear, therigidity characteristic of the brake pad 21 can be estimated by storingthe relation among the pad temperature, the amount of wear and therigidity characteristic in advance as shown in FIG. 6-(C). In FIG.6-(C), (a) and (b) show rigidity characteristics in the case that theamount of wear is small, in which (a) shows a rigidity characteristic ata low temperature and (b) shows a rigidity characteristic at a hightemperature. The brake pad 21 becomes soft at a higher temperature, andthe increase of the thrust of the piston relating to the piston positionbecomes milder. In FIG. 6-(C), (c) and (d) show rigidity characteristicsin the case that the amount of wear is large, in which (c) shows arigidity characteristic at a low temperature and (d) shows a rigiditycharacteristic at a high temperature. When the amount of wear is large,the amount of the brake pad to be deformed reduces accordingly. As aresult, the increase of the piston thrust relating to the pistonposition becomes sharp, and the change of the rigidity characteristicrelating to the temperature change becomes small. In this way, bystoring the rigidity characteristic of the brake pad 21 as a function ofthe temperature and the amount of wear, the rigidity characteristic canbe estimated.

If the temperature of the brake pad 21 can be estimated, thrust controldepending on the change of the coefficient of friction of the pad can berealized. A correction gain with respect to a target value of the pistonthrust is set in advance so that the change of the coefficient offriction depending on the pad temperature does not affect thedeceleration rate of the vehicle. The change in deceleration rate of thevehicle due to the change in temperature of the pad can be suppressed bysetting the thrust of the piston to be large at such a pad temperaturethat the coefficient of friction is small, and setting the thrust of thepiston to be small at such a pad temperature that the coefficient offriction is large. As shown in FIG. 7-(A), the thrust of the piston iscorrected by multiplying a control target value for controlling thethrust of the piston by a correction coefficient having invertedrelation to the friction coefficient with respect to the temperature.FIG. 7-(B) shows time responses in the case where lowering of the padtemperature is detected at the time of starting the braking control.This example assumes the state where the estimated value of the padtemperature is T1 before the piston reaches the braking start position,while the pad temperature lowers to T2 at time t1. A pad gap is securedat the time of releasing the brake, and the braking control is startedat the time t1. When the braking start position is updated at time t2,the estimated value of the pad temperature is updated to T2 which islower than T1. If the estimated value of the pad temperature remains atT1, the time response will be that shown in (a) of FIG. 7-(B), but sincethe estimated value of the pad temperature is updated to T2, the timeresponse changes to that shown in (b) of FIG. 7-(B). Because thecoefficient of friction of the brake pad 21 is lower at the temperatureT2, the correction gain G2 higher than the correction gain G1 at thetemperature T1 is obtained. Accordingly, the rising gradient of thethrust of the piston becomes large, and the convergence value alsoshifts to F2 which is higher than F1 at temperature T1.

In addition, if the amount of thermal expansion and the temperature ofthe pad can be estimated, the rigidity of the pad can be estimated, andthe thrust of the piston can be controlled without using a thrustsensor. As shown in FIG. 6-(C), the rigidity of the brake pad 21decreases as the temperature increase, and increases as the temperaturedecreases. If the change in the thrust of the piston to the pistonposition as shown in FIG. 6-(C) is stored in advance, a piston positioncorresponding to a desired value of the thrust of the piston can bedetermined, and therefore the piston thrust control can be achieved byperforming the feed-back control of the piston displacement withoutusing a thrust sensor.

1. An electric brake apparatus comprising: a disc rotor rotatingtogether with a wheel; an actuator for rectilinearly actuating a pistonin an axial direction of the disc rotor by using an electric motor; adrive controller for performing drive control of the actuator; a brakepad to be pressed by the piston to give frictional resistance to thedisc rotor in a rotational direction; and a braking start positiondetecting means for detecting a braking start position of the piston atwhich position the brake pad is brought into contact with the discrotor, characterized in that the drive controller stores the brakingstart position detected by the braking start position detecting means asa maximum braking start position, and updates a stored value of themaximum braking start position when the braking start position shifts ina pressing force increasing direction.
 2. The electric brake apparatusaccording to claim 1, characterized in that the drive controller updatesthe braking start position detected by the braking start positiondetecting means as a current braking start position at intervals shorterthan those of the maximum braking start position, and calculates thethermal expansion amount of the brake pad based on the differencebetween the maximum braking start position and the current braking startposition.
 3. The electric brake apparatus according to claim 1,characterized in that the drive controller stores a braking startposition at the time of assembling the brake pad as an initial brakingstart position, and calculates the wear amount of the brake pad based onthe difference between the maximum braking start position and theinitial braking start position.
 4. The electric brake apparatusaccording to claim 2, characterized in that the drive controller changesthe thrust of the piston or the position of the piston based on thedifference between the maximum braking start position and the currentbraking start position, or the difference between the maximum brakingstart position and the initial braking start position.
 5. The electricbrake apparatus according to claim 2, characterized in that the actuatorcomprises a parking brake mechanism for mechanically holding theposition of the piston, and the drive controller changes the thrust ofthe piston at the time of holding the position of the piston by theparking brake mechanism based on the thermal expansion amount of thebrake pad.
 6. The electric brake apparatus according to claim 2,characterized in that the actuator comprises a parking brake mechanismfor mechanically holding the position of the piston, and the drivecontroller changes the position of the piston at the time of holding theposition of the piston by the parking braking mechanism based on themaximum braking start position.